1. Field of the Invention
The present invention relates to an overheat detection circuit to be used for a power supply apparatus, or the like.
2. Description of the Related Art
FIG. 7 is a circuit diagram for illustrating a related-art overheat detection circuit to be used for a power supply apparatus or the like. The related-art overheat detection circuit includes a reference voltage circuit 201, a temperature sensing circuit 202, a comparator circuit 203, a power supply terminal 300, and a ground terminal 301.
The reference voltage circuit 201 includes NMOS transistors 101 and 102, PMOS transistors 103, 104, and 105, and resistors 110 and 111. The temperature sensing circuit 202 includes a PMOS transistor 106 and a diode 120.
The NMOS transistor 101 has a gate and a drain connected to each other, and has a source connected to the ground terminal 301. The NMOS transistor 102 has a gate connected to the gate of the NMOS transistor 101. The resistor 110 is connected between a source of the NMOS transistor 102 and the ground terminal 301. The PMOS transistors 103, 104, and 105 form a current mirror circuit. The resistor 111 is connected between a drain of the PMOS transistor 105 and the ground terminal 301. Then, a reference voltage Vref is output from a contact point (reference voltage output terminal) of the resistor 111 and the PMOS transistor 105. In this case, the resistor 110 and the resistor 111 have the same temperature coefficient.
The PMOS transistor 106 and the PMOS transistor 103 form a current mirror circuit. The diode 120 serving as a heat sensitive element is connected between a drain of the PMOS transistor 106 and the ground terminal 301. Then, a forward voltage of the diode 120, namely, a temperature voltage Vf is output from a contact point (temperature voltage output terminal) of the diode 120 and the PMOS transistor 106.
The reference voltage Vref is input to an inverted input terminal of the comparator circuit 203 and the temperature voltage Vf is input to a non-inverted input terminal of the comparator circuit 203. The forward voltage of the diode, namely, the temperature voltage Vf generally has a negative temperature coefficient of approximately −2 mV/° C. when a parasitic diode of a CMOS process is used. The reference voltage Vref is set to be equal to the temperature voltage Vf corresponding to a temperature to be detected. The overheat detection circuit is configured to compare the reference voltage Vref and the temperature voltage Vf with each other to obtain a magnitude relationship thereof through use of the comparator circuit 203, to thereby detect an overheat state.
When detecting the overheat state, the comparator circuit 203 outputs a detection signal Vdet and controls an output transistor and the like of the power supply apparatus based on the detection signal Vdet, to thereby protect the power supply apparatus from the overheat.
However, the related-art overheat detection circuit is required to include the reference voltage circuit, the temperature sensing circuit, and the comparator circuit, and thus has a large circuit scale. Further, there is a problem in that each circuit constantly consumes a current and thus it is difficult to reduce the electric power consumption.